In the days before he stole a semiautomatic weapon and stormed into an Oregon shopping mall, killing two people in a shooting spree, Jacob Roberts quit his job, sold his belongings and began to seem "numb" to those closest to him.

Roberts' ex-girlfriend, Hannah Patricia Sansburn, 20, told ABC News today that the man who donned a hockey mask and opened fire on Christmas shoppers was typically happy and liked to joke around, but abruptly changed in the week before the shooting.

Roberts unleashed a murderous volley of gunfire on the second floor of the Clackamas Town Center on Tuesday while wearing the mask and black clothing, and carrying an AR-15 semiautomatic weapon and "several" magazines full of ammunition. He ended his barrage by walking down to the first floor of the mall and committing suicide.

"I don't understand," Sansburn said. "I was just with him. I just talked to him. I didn't believe it was him at all. Not one part of me believed it."

She said that in recent weeks, Roberts quit his job at a gyro shop in downtown Portland and sold all of his belongings, telling her that he was moving to Hawaii. He had even purchased a ticket.

She now wonders if he was really planning to move.

"He was supposed to catch a flight Saturday and I texted him, and asked how his flight went, and he told me, 'oh, I got drunk and didn't make the flight,'" she said. "And then this happens... It makes me think, was he even planning on going to Hawaii? He quit his job, sold all of his things."

Roberts described himself on his Facebook page as an "adrenaline junkie," and said he is the kind of person who thinks, "I'm going to do what I want."

Roberts, who attended Clackamas Community college, posted a picture of himself on his Facebook page firing a gun at a target. His Facebook photo showed graffiti in which the words "Follow Your Dreams" were painted over with the word "Cancelled."

Sansburn said the pair had dated for nearly a year but had broke up over the summer. Throughout their relationship, she had never seen him act violently or get angry.

"Jake was never the violent type. He didn't go out of his way to try to hurt people or upset people. His main goal was to make you laugh, smile, make you feel comfortable. I never would have guessed him to do anything like this ever," she said.

"You can't reconcile the differences. I hate him for what he did, but I can't hate the person I knew because it was nothing like the person who would go into a mall and go on a rampage. I would never associate the two at all."

The last time she saw him, which was last week, he "seemed numb," and she didn't understand why, she said.

"I just talked to him, stayed the night with him, and he just seemed numb if anything. He's usually very bubbly and happy, and I asked him why, what had changed, and said 'nothing.' He just had so much he had to do before he went to Hawaii that he was trying to distance himself from Portland," Sansburn said.

Sansburn said the last message she sent Roberts was a text, asking him to stay, and saying she didn't want him to leave. He replied "I'm sorry," with a sad face emoticon.

Police are still seeking information about what Roberts was doing in the days leading up to the shooting. They said today they believe Roberts stole the gun he used in the rampage from someone he knew. They have searched his home and his car for other clues into his motive.

Clackamas County Sheriff Craig Roberts said earlier today on "Good Morning America" that he believes Roberts went into the mall with the goal of killing as many people as he could.

"I believe, at least from the information that's been provided to me at this point in time, it really was a killing of total strangers. To my knowledge at this point in time he was really trying, I think, to kill as many people as possible."

Sansburn said she has not talked to police.

Mall Shooter Told Ex-Girlfriend He Was Moving to Hawaii

Roberts' mother, Tami Roberts, released a statement through a friend today saying she had no explanation of her son's behavior.

"Tami Roberts wishes to express her shock and grief at the events at Clackamas Town Center Tuesday," the friend said, reading from a statement outside of Tami Roberts' home.

The friend noted that Tami raised Roberts, but was not his biological mother. Sansburn said that Roberts' biological mother had died when he was very young.

"She has no understanding or explanation for her son's behavior...It's so out of his character," the friend said.

Officials from the Oregon City School District, where Roberts graduated from Oregon City High School, were also surprised at the news.

"This news is very shocking to those who knew Jacob while at OCHS. He was known as a soft spoken and polite young man who was often eager to be helpful. The motive for such a horrific act is likely to remain a mystery to us all," the district said in a statement.

Roberts had attended the school for only his senior year. He spent three years at Milwaukie High School, where officials described him as an "average student with average grades."

He had no disciplinary record at the school, but chose to transfer for his senior year to Oregon City High School, according to Joe Krumm, an administrator at the North Clackamas School District. Krumm did not know why he transferred, and said that in all, Roberts did not stand out in memory for anything in particular.